In semiconductor fabrication, photoresist (or resist) must be removed following a wide variety of processing steps, including etching (wet and dry), ion implantation, lift-off processes, high temperature postbake (for improving resist adhesion or etch resistance), or merely simple removal of misaligned resist patterns for reimaging after development and inspection ("rework"). In addition, wafer surface patterns of several different materials may be present under the resist (e.g. SiO.sub.2, aluminum, polysilicon, suicides, deposited SiO.sub.2 or Si.sub.3 N.sub.4 or polyimide). The main objective in resist stripping is to insure that all the photoresist is removed as quickly as possible without attacking any underlying surface materials. Resist stripping techniques are generally divided into three classes: 1) organic strippers; 2) oxidizing-type (inorganic) strippers; and 3) dry type stripping techniques. Dry etching of resist is done using oxygen plasmas in plasma etching equipment. Dry etching offers several advantages over wet resist strippers including safer operating conditions, no metal ion contamination, reduced pollution problems, and less attack of most underlying substrate materials.
The density of devices fabricated on semiconductor substrates has increased steadily over the years with ultra large scale integration. Accompanying this trend have been decreased feature sizes and increased demands on process technology. To pattern such small features, conventional lithographic procedures are being supplanted by newer ones based on diffusion enhanced silylated resist DESIRE.TM. processes. Diffusion enhanced silylated processes can produce sub-half micron features in various resists, using one line and deep ultra violet light exposure. The resolution and throughput rate up to the image transfer step exceeds that of conventional positive resists and are clearly superior when topography is of major concern.
The resist is somewhat more difficult to remove with diffusion enhanced silylated resist processes as compared with conventional processes as a result of larger amounts of etch byproducts such as sidewall polymer on vertical walls of a device undergoing fabrication. These byproducts, generally referred to as polymers are generally comprised of a metal and SiO.sub.2 molecule. For instance, the molecule can comprise carbon from the photoresist, metal from the metal layer and SiO.sub.2. Further, sidewall polymers may comprise aluminum silicate and very small amounts of fluorocarbons. Fluorocarbons are non-combustible and therefor are not removed during an O.sub.2 in-situ ash sequence of a metal etch. Thus, ashing has proven to be ineffective because of the high carbon content in the byproduct molecule from the photoresist. The difficulty with which resist can be removed has proven to be a sever impediment to the generation of sub-half micron features. Previously solvent/ultrasonic agitation had been used to remove SWP. However, these techniques prove to be unusable because of the tendency of metal, such as aluminum, to lift off of the minimum features. Further, these techniques tend to leave behind significant amounts of residue on device sidewalls and on device surfaces.
One of the major challenges facing state-of-the-art backend process technologies is the requirement for complete removal of polymers generated during etch of high aspect ratio via holes. The etching processes required to control via sidewall profiles and maintain high selectivity to mask and substrate usually generate polymers which are very difficult to remove. Residual polymers that are not removed will result in vias having higher resistances. These polymers are typically composed of both organic and inorganic components and may required both dry and wet stripping to achieve complete removal. The more difficult the polymer is to remove, the more aggressive must be the solvent stripper, with consequences in safety, cost, and manageability. The burden of strippability can be lessened by making the post-etch ash more effective in creating a soluble polymer, and by increasing the dry etch selectivity to the substrate material.
In view of the deficiencies in conventional methods, the art is in search of improved techniques for removing sidewall polymers and particularly improved methods for forming vias having lower resistances.